Photomasks are used in photolithographic processes for printing electrical circuit wiring patterns and other precision photofabricated parts. In a typical photolithographic process, a substrate is covered with a layer of photoresist material over which a photomask is superimposed. The photomask has a pattern of opaque and transparent areas with respect to actinic radiation, typically ultraviolet light, which is passed through the photomask to reproduce the pattern in the photoresist material. The pattern is developed as a relief image in the photoresist material by means of different solubilities of the exposed and unexposed portions of the photoresist material.
Since the preparation of a photomask requires a substantial amount of time, labor and material, it is desirable that a photomask be sufficiently durable for repeated use in the manufacture of photofabricated articles. It is also desirable to maximize the resolution of the pattern carried by a photomask in order to improve the precision of the image it transfers to the photofabricated articles.
Photolithographic processes have employed photomasks comprising a sheet of glass carrying a patterned coating of chromium, iron oxide or photographic emulsion. While iron oxide and chromium films are considerably more durable than photographic emulsions, all coated photomasks are subject to scratching and other mechanical damage which shortens their useful life. In addition, the etching required to produce a desired pattern in chromium or iron oxide films produces a loss of resultion as a result of the etch factor, the fact that an etched groove grows wider as it grows deeper.
Photomasks of improved durability comprising a stained pattern within a glass substrate are disclosed in U.S. Pat. No. 3,573,948 to Tarnopol and U.S. Pat. No. 3,732,792 to Tarnopol et al. Although these stained glass photomasks have improved durability, the step of etching a pattern through a stained layer of the glass in the former or the step of etching through a tin oxide coating in the latter results in insufficient resolution for some applications. U.S. Pat. No. 3,561,963 to Kiba discloses a stained glass photomask wherein the desired pattern is etched into a copper film on a glass substrate, and copper irons are subsequently migrated into the glass by heating. Although the stained photomask pattern is more durable than a coating, resolution is compromised in this process as a result of the etching of the film and the migration step which results in lateral spreading of the stained areas into the adjacent unstained areas.
U.S. Pat. No. 2,927,042 to Hall et al. and U.S. Pat. No. 3,620,795 to Kiba disclose methods designed to minimize the lateral diffusion of staining ions in the aforementioned processes. The Hall patent describes depositing a film of stain-producing metal onto glass and removing portions of the film by photoetching. An electrical field is then passed through the glass so that the patterned film migrates into the glass substrate. The Kiba patent discloses etching a pattern into a metal film and migrating stain-producing ions through aperture in the metal film by heating in an electric field. Both methods suffer a loss of resolution as a result of the etching step. U.S. Pat. Nos. 2,732,298 and 2,911,749 to Stookey both disclose the production of a stained image within a glass plate by heating a developed silver-containing photographic emulsion on the glass. However, the use of relatively high temperatures of 400.degree. to 650.degree. C. results in a loss of resolution of the stained pattern, and the obtainable optical density is not as high as may be required.
U.S. Pat. No. 4,155,735 to Ernsberger discloses an improved method for making stained glass photomasks. The method comprises developing a patterned photoresist layer on a glass substrate and applying an electric field to enhance the migration of staining ions through apertures in the photoresist pattern into the surface of the glass substrate. The staining ions are then reduced and agglomerated to form a stained pattern within the surface of the glass by heating the glass in the presence of a reducing agent such as tin or copper ions, or in a reducing atmosphere such as forming gas, preferably at temperatures of 400.degree. to 500.degree. C.
In U.S. Pat. No. 4,309,495, Ernsberger describes producing stained glass photomask patterns by exposing and developing a photographic emulsion on a sheet of glass and migrating silver ions from the emulsion into the surface of the glass under the influence of an electric field and moderately elevated temperatures. These silver ions are then reduced and agglomerated to form a stained pattern with the surface of the glass by maintaining the glass at an elevated temperature in the presence of a reducing agent. The reducing agent may be reducing ions such as cuprous ions migrated into the glass, or the stannous ions inherently present near the surface of glass produced by the float process, in which case an optimized rate may be obtained at temperatures of 475.degree. to 525.degree. C. In an alternative embodiment, the reducing agent may be a reducing atmosphere such as forming gas in the heating chamber during the reducing and agglomerating heat treatment, in which case practical rates can be realized at lower temperatures in the range of 350.degree. to 400.degree. C.
Microscopic examination of stained patterns in glass photomasks made in accordance with the aforementioned methods shows that the pattern edges are slightly blurred. A microdensitometer scan shows that the optical density profile of such an edge is sloped, requiring as much as fifteen microns (about 0.006 inches) to go from maximum to minimum density. This sloping profile is known as the "roll-off region". The ideal optical density profile of an edge, which could be described as perfect edge definition, would be rectangular. For certain purposes, such as photomasks intended for use in the silicon integrated circuit industry, pattern lines only five to ten microns wide are necessary. Therefore, edge definition must be very good. A roll-off region of one micron width may be the upper tolerable limit.
Microscopic examination of stained patterns in glass photomasks made in accordance with the aforementioned methods also shows that when the reduction and agglomeration of stain-producing ions are carried out in a forming gas atmosphere at high temperatures, the stained pattern contains a significant portion of the reduced stain-producing ions in an over-agglomerated form; that is, as typically spherical particles of microscopically-resolvable size. This method of producing a stained pattern is therefore somewhat inefficient, since stain particles of such dimensions contribute very little to the ultraviolet radiation absorption band, which is produced predominantly by the resonant light absorption of particles of submicroscopic colloidal dimensions. The magnitude of this inefficiency can be appreciated from the fact that one spherical particle one micrometer in diameter would contain enough silver, for example, to yield a million particles of the preferred diameter, 0.01 micrometers. Thus, a given quantity of silver per unit area will provide a much greater absorbance (optical density) in the desired spectral region of ultraviolet radiation if agglomeration is effected under conditions which yield submicroscopic particles.